Fully-heavy tetraquark spectroscopy in the relativistic quark model

TL;DR

This paper calculates the masses of fully-heavy tetraquarks using a relativistic quark model, identifying potential matches with recently observed exotic resonances and providing insights into their structure and decay properties.

Contribution

It presents a comprehensive relativistic quark model calculation of fully-heavy tetraquark spectra, including excited states and decay thresholds, with implications for interpreting recent experimental discoveries.

Findings

Most tetraquark states lie above decay thresholds, likely appearing as broad resonances.

The narrow X(6900) resonance is identified as an excited fully-charmed tetraquark.

Other exotic resonances may correspond to different excitations of fully-charmed tetraquarks.

Abstract

Masses of the ground and excited (1P, 2S, 1D, 2P, 3S) states of the fully-heavy tetraquarks, composed of charm ($c$) and bottom ($b$) quarks and antiquarks, are calculated in the diquark-antidiquark picture within the relativistic quark model based on the quasipotential approach and quantum chromodynamics. The quasipotentials of the quark-quark and diquark-antidiquark interactions are constructed similarly to the previous consideration of mesons and baryons. Relativistic effects are consistently taken into account. A tetraquark is considered as a bound state of a diquark and an antidiquark. The finite size of the diquark is taken into account, using the form factors of the diquark-gluon interaction. It is shown that most of the investigated states of tetraquarks lie above the decay thresholds into a meson pair, as a result they can be observed only as broad resonances. The narrow state X(6900) recently discovered in the di-$J/\psi$ production spectrum by the LHCb, CMS and ATLAS Collaborations corresponds to an excited state of the fully-charmed tetraquark. Other recently discovered exotic heavy resonances X(6200), X(6400), X(6600), X(7200), X(7300) can also be interpreted as the different excitations of the fully-charmed tetraquark.